Input cap (2.2uF) / 50k pot (bass roll-off) question: - diyAudio
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Old 23rd September 2003, 01:43 AM   #1
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Question Input cap (2.2uF) / 50k pot (bass roll-off) question:

I had some decent quality 2.2uF caps that I decided to use for input caps in an LM3875-based IGC.

I am pretty pleased with the results, but I have only heard them on a (crummy) set of Yamaha 3-way speakers. (I will admit that part of me actually hoped the IGC would have enough of a DC offset problem to kill them and force me to buy something better for my 2nd set of speakers!)

Anyway, I am not totally pleased with the bass and am narrowing down causes. It may very well be the Yamaha's, but they seem to have more bass with my (don't laugh) Sony receiver that is the heart of my (soon to be replaced by DIY) 2nd system.

Back to my question. I used 2.2uF input caps (instead of 4.7uF) since I had them laying around. I did not think that would roll off the bass too high, but I may be wrong. Wouldn't be the first time.

I am using a 50K pot on the input. Does that mean I have a 50Kohm input impedance? Would that, combined with a 2.2uF input cap, cause me to be rolling off my bass in the audible range?

I would try it on my good speakers (JMLab Chorus 707's) but I have not had the time to go through and solder all of my DC connections (they are crimp on spades for the temporary board version.) A loose connection gave me a 185mv DC offset when I hooked it up at first in the right channel, so no JMLabs until the soldering is done up and down both rails.

Could that cap/pot combo be my problem? Or should I move on and look for other culprits which could, of course, be the speakers.

No opinions unappreciated!

Thanks,

GnD
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Old 23rd September 2003, 05:25 PM   #2
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Old 23rd September 2003, 05:52 PM   #3
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you can work out the -3dB pont quite easily for this, if you multiply the value of the capacitor by the value of the input resistance and then that value by two times pi. Then take that value and divide one by it you will get your -3db frequency ( sorry that is about as clear as a brick, if you want ask and I will go through it again). So, assuming you don't have a low value resistor to ground after your volume pot, the -3dB point of your system would be at about 1.4hZ which should be ok for most sets of speakers.
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Old 23rd September 2003, 06:15 PM   #4
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-3dB point ( in Hz ) = 1/(2 xpi x C x R )

1/( 2 x pi x 2.2exp10-6 x 50000 ) = 1.448 Herz

This assuming the input resistance of the chip is infinite which it isn't. Check the datasheet for that and parallel that resistance to that of the potentiometer.

Suppose the chips input impedance is 50 k as well ( just for the calculation ).

1/ ( 2 x pi x 2.2exp10-6 x 25000 ) = 2.89 Herz.

So it's pretty obvious the cap's value is not the problem. Maybe oscillation is .

When not it can be the subjective impression of the cap/potentiometer combination. Search for oscillation first as it will very likely be the problem. Please consider the tips Joe Rasmussen gave for wiring and/or the new revised schematic on the Amp chip forum.
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Old 23rd September 2003, 06:36 PM   #5
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Thanks to both of you for the pointers... but how does one tell if a chip is oscillating?
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Old 23rd September 2003, 06:49 PM   #6
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If used, as GnD said, with IGC (Inverted GainClone), and with pot at the minimum position (or at maximum position assuming zero source impedance), the impedance that 2.2uF input cap sees is mainly Rin, which is usually 10k. Hence, at 20Hz roll-off will be about 0.5dB and this is followed by about 20° phase shift. The -3dB (and 45° phase shift) point will be at 7.2Hz. In all other cases the whole slope goes lower.

So, I’d say 2.2uF is more or less acceptable as an input cap for IGC.

Quote:
Originally posted by GrahamnDodder
Could that cap/pot combo be my problem?
No.

Quote:
Or should I move on and look for other culprits…
Yes.

Quote:
…which could, of course, be the speakers.
This hardly has anything with speakers.


Pedja
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Old 23rd September 2003, 09:32 PM   #7
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Quote:
If used, as GnD said, with IGC (Inverted GainClone), and with pot at the minimum position (or at maximum position assuming zero source impedance), the impedance that 2.2uF input cap sees is mainly Rin, which is usually 10k. Hence, at 20Hz roll-off will be about 0.5dB and this is followed by about 20° phase shift. The -3dB (and 45° phase shift) point will be at 7.2Hz. In all other cases the whole slope goes lower.
Right, my Gainclone to be is non inverting so I made a wrong assumption. GnD clearly wrote IGC. The outcome is nevertheless practically the same, it is not the cap value that causes the problem. But it won't hurt to use a 4.7 uF cap if you want to try it. Do check for oscillation with an oscilloscope . You won't be the first with an oscillating (I)GC.

No signal in and still signal ( mostly higher frequencies or even RF ) at the output means it oscillates.
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